Diode sputtering is a commonly used method of coating objects with a thin film of material. This method of material deposition has been used in the past for a number of applications including semiconductors, superconductors, and optical coating.
Generally diode sputtering first requires the creation of a gas discharge within a chamber. Operation of the gas discharge causes an exchange of electrons and ions between a cathode and an anode. Collision of ions with the cathode, or a target placed in front of the cathode causes material to be dislodged from the cathode and thus deposited on other objects within the chamber. Typically, the substrate or object to be coated is placed directly in front of the cathode. Because of its positioning in front of the cathode, material dislodged from the cathode is deposited on the substrate surface at a very high rate.
In a typical gas discharge a plasma exists between the cathode and the anode. The plasma is a collection of ions and electrons that have an overall neutral charge. During operation of the discharge ions are accelerated towards the cathode surface. Their collision with the cathode surface causes material to be released from the cathode surface, and, occasionally, a negatively charged ion is released. Due to the negative charge on the ion and the negative charge of the target, the negatively charged ion is accelerated away from the target at a very high rate. These highly accelerated ions pass through the plasma and collide with any objects that are directly in their path. Secondary electrons are also emitted from the surface being sputtered. These electrons are also accelerated across the dark space and can cause heating of objects in their path.
In the past the substrate has been placed directly in front of the cathode surface. Consequently, when negatively charged ions, as well as electrons, are released from the cathode surface and pass through the plasma they collide with the substrate surface. Due to the high energy these ions possess, their collision with the substrate surface is often very destructive, resulting in damage and imperfection of the thin film coating. The electrons can cause excessive heating of the substrate and undesirable film growth.
Diode sputtering is often used to fabricate high quality mirrors. These high quality mirrors then are used for such applications as lasers and ring laser gyroscopes. As is well known in the art, the mirrors are fabricated by depositing alternating layers of material upon a substrate. High quality mirrors are achieved by having each of these coatings be very uniform and of high quality.
Lastly, it is advantageous to produce a large quantity of mirrors at one time. This becomes very complicated since alternating layers of material are required for fabrication of a mirror. Gas purity and cathode material purity are a requirement for quality sputtering, consequently, it is necessary to provide a method to coat the substrate with alternating layers of material without opening and closing the chamber constantly.